Falsifiability is the ability of a theory—a working framework for explaining and predicting natural phenomena—to be disproved by an experiment or observation.[1] The ability to evaluate theories against observations is essential to the scientific method, and as such, the falsifiability of theories is key to this and is the prime test for whether a proposition or theory can be described as scientific. Put simply, if a theory cannot be falsified, there is no point in even examining the evidence.

All scientific knowledge and theories are based on two things: observation and consistent logic. A theory is a logical explanation for observations. A good, scientific theory also proposes a set of new observations that could test a theory's power to explain. Once technology, time or funding catches up with the theory, these observations can be made, which can either support or invalidate the theory. This ability to be tested, and the potential for the theory to be invalided by the experiment, is the essence of falsifiability.

It has been argued, most notably by Karl Popper, that the scientific method demands that a theory must at least in principle be falsifiable in order for it to be valid as science. This requirement was Popper's solution to the demarcation problem, or what is and what is not science. Popper's view is not widely acceptable in contemporary philosophy of science. However, that does not mean that falsifiability is not important. Falsifiability is a virtue in a scientific theory. Evolution, for example, is theoretically falsifiable - "fossil rabbits in the Precambrian", as J.B.S. Haldane once said - whereas intelligent design is not, mostly because it makes no predictions that can actually be tested.

Newton's laws allowed us to make specific predictions regarding such things as the trajectories of artillery projectiles or the orbits of planets. These predictions were observed to hold most of the time, but in some cases such as the orbit of Mercury there were observations that contradicted predictions based on Newton's laws. This led to their modification and replacement by relativity, which, rather than being a complete rejection of the Newton's laws, was a clarification and refinement that allowed them to hold true in a greater range of observable circumstances. It was the specific predictions made by Newton's laws that allowed scientists to test them, and eventually replace them. This happened because the predictions made Newton's laws falsifiable.

A simple procedure can be used to determine whether or not a hypothesis or conjecture is scientific and falsifiable. What would be an example of something that, if observed, would contradict the hypothesis? If this question cannot be answered, then the conjecture is not scientific. In addition, a good test of a theory is that it is able to make predictions about some future event. For example, Einstein's ideas about relativity predicted specific things that would be observed during a total solar eclipse. When the eclipse came, the predictions were confirmed, something which strongly supported his theory.

Logically, the two applications are the same thing. A theory useful in practice is not defined by what it allows, but by what it disallows, because that is where its predictive power lies. For example, Newton's theory of gravity says that the force of universal attraction cannot act in any other way except in accordance to the inverse square formula. If a theory allows anything whatsoever to happen (e.g. via Goddidit), it can make no useful predictions, and conversely, if a theory says that an event cannot happen, then such an event would falsify the theory — and practical uses of the theory can rely on the fact that this event will never happen.

Creationism is not falsifiable as its proponents base the conjecture on a human text (the Bible) which provides accounts of creation and other events that cannot be tested by observation or experiment but are instead accepted as infallible truth. This is one of the primary characteristics of pseudoscience. No matter what evidence is presented, there is no way that creationism can be contradicted. Even when evolution in action is observed, creationism always allows for an after-the-fact justification of the inconsistent observation with an argument to authority. Put differently, for any possible observation you can imagine Creationism can explain away both that observation and its opposite. Only an observation proving that God does not exist would undermine the theory, and obviously that is impossible. Since no observation is allowed to contradict creationism and it has no predictive value, it is not science.

It is important to remember that just because a theory is falsifiable it may never be falsified, contrary to some anti-evolutionist thought. Theory is neither the opposite of "fact" nor a synonym for "hunch" or "hypothesis". "Theory" is a scientific term of art; a theory is a hypothesis which has been able to withstand repeated falsification attempts to the point of being generally accepted by the practicing scientific community. Even though details about the exact way evolution occurred are debated, evolution is almost universally accepted by scientists. Despite this fact, scientists would reconsider evolution if suddenly observations seemed to contradict its principles and falsify the idea. Note however that when a new observation contradicts a well established theory it is usually rational to treat the single observation, not the established theory, as suspect (unless and until it can be verified and replicated).

Willingness to reexamine facts objectively is the key difference between a scientist and a theologian.

The Duhem-Quine Thesis is something of a conflation of the ideas of Pierre Duhem and W.V.O. Quine. In short, it states that it is impossible to test or falsify a hypothesis in isolation for two reasons. One, the hypothesis relies on a number of supporting assumptions. For example, does the exposure of the Piltdown Man fraud falsify the validity of that fossil alone or all of evolutionary theory? Two, a discrepancy between theory and data does not necessarily falsify the theory. The data itself may have been collected in an erroneous fashion. The Duhem-Quine Thesis is often contrasted with or considered to modify "naive" or "Popperian" falsificationism.[2]

Imre Lakatos further extended Popperian falsification and the Duhem-Quine Thesis with his concept of "research programs." Lakatos defined extensively developed theories and techniques in a field as the "hard core" of a research program. Around this hard core is a "protective belt" of auxiliary hypothesis and modified or ad hoc assumptions added by scientists to protect the core of the program from falsification. While the protective belt is auxiliary and ad hoc, this does not necessarily translate to "bad" in Lakatos' view. If the protective belt leads to the discovery of new facts made by novel predictions, this leads to what Lakatos called a "progressive research program." A "degenerative research program" is one in which the protective belt grows but leads to no new discoveries, merely acting as a way to cover up the flaws of the hard core.[3][4]